I feel like this is a subject that keeps getting beat to death.

It doesn't matter how many people are or aren't on each server. The idea is to make as many as possible, because it sells the game as super-popular.

How?

Easy. Noobs are always directed to the front of the wave. So whether the 100 servers behind them are dead or not, most will never be aware because they will be where the action is, so there's an appearance of fullness and growth. By the time the server dies, there's at least a few people left too invested to leave. But at least there are a hundred "full" servers out there for the casual observer. In a short time that will be several hundred.